Particle Acceleration and Magnetic Field Amplification by Relativistic Shocks in Inhomogeneous Media

Abstract

Particle acceleration and magnetic field amplification in relativistic shocks propagating in inhomogeneous media are investigated by three-dimensional magnetohydrodynamical (MHD) simulations and test-particle simulations. The MHD simulations show that the interaction between the relativistic shock and dense clumps amplifies the downstream magnetic field to the value expected from observations of the gamma-ray burst. The test-particle simulations in the electromagnetic field given by the MHD simulation show that particles are accelerated by the downstream turbulence and the relativistic shock. We provide the injection energy to the shock acceleration in this system. If the amplitude of upstream density fluctuations is sufficiently large, low-energy particles are initially accelerated to the injection energy by the downstream turbulence and then rapidly accelerated to higher energies by the relativistic shock. Therefore, the density fluctuation significantly affects particle acceleration in the relativistic shock.

Collisionless Shock in a Relativistically Hot Unmagnetized Electron–Positron Plasma

“…It takes about the eddy turnover time to amplify the magnetic field by turbulent; that is, the turbulent dynamo cannot make a magnetic field turbulence just behind the main shock. Then, low-energy particles with a small gyroradius cannot be injected to particle acceleration by the main shock (Morikawa et al 2024). On the other hand, our mechanism proposed in this section can quickly generate the magnetic field turbulence in the kinetic scale by the Weibel Figure 7.…”

“…The magnetic field generation may also work even in the internal shock. Some mechanisms of the magnetic field generation or amplification in GRBs have been proposed in the kinetic (Chang et al 2008;Spitkovsky 2008a;Keshet et al 2009;Tomita & Ohira 2016;Tomita et al 2019;Grošelj et al 2024) and magnetohydrodynamical scales (Sironi & Goodman 2007;Inoue et al 2011;Mizuno et al 2011;Tomita et al 2022;Morikawa et al 2024).…”

“…In addition to the magnetic field generation, the downstream shock would be important for particle acceleration. The interaction between high-density regions and the main shock drives the downstream turbulence, so that the downstream magnetic field can be amplified by turbulent dynamo (Sironi & Goodman 2007;Inoue et al 2011;Mizuno et al 2011;Tomita et al 2022;Morikawa et al 2024). It takes about the eddy turnover time to amplify the magnetic field by turbulent; that is, the turbulent dynamo cannot make a magnetic field turbulence just behind the main shock.…”